Assessment of groundwater potential in fractured hard rocks around Vryburg, North West Province, South Africa

Expanded Title:

In this study, investigation of groundwater potential in crystalline basement rocks of the North West Province was carried out. The area of study is located in the Naledi Local Municipality situated in the central part of the North West Province. It covers an area of ~7260 km2 and consists of 8 Quaternary catchments. Hydrogeologically, a large part of the area falls within the Lower Vaal catchment. The average annual precipitation in the area is ~350 mm and temperature varies from very cold (below freezing point) to 350 C during the warm seasons. Groundwater recharge in the area is low (<10 mm) and largely depends on temperature and the seasonality and intensity of rainfall. Potential evaporation rate ranges from 1960 mm to 2100 mm per annum, exceeding annual rainfall. It is typically a semi-arid to arid region, and groundwater is the main source of water supply for domestic and agricultural use. The resulting map shows a number of groundwater potential zones varying from “very good” to “very poor”. The zone shown as ‘very good’ and ‘good’ groundwater potential covers ~17% and ~22% of the study area, respectively. The superimposed borehole yield also confirms the results derived from multivariate statistical modeling approaches, whereby high borehole yields (> 15 l/s) fall within carbonate rocks consisting of dolomite and limestone located in the southern part of the area. In addition, follow-up geophysical surveys carried out at selected sites confirmed the presence of conductive layers varying in depth from 20 m to 35 m. The high conductivity possibly indicates the presence of water-bearing formation, in particular dolomite located around Vryburg and highly fractured granite just south of Stella. The high yielding wellfields can be attributed to dissolution of carbonate rocks by water that percolates through pre-existing fractures leading to enlarged fracture apertures, and consequently resulting in the development of large cavities that can store and supply significant amount of water.